Phys. Rev. D 95, 072004 (2017) - Measurement of higher-order multipole amplitudes in $\ensuremath{\psi}(3686)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{c1,2}$ with ${\ensuremath{\chi}}_{c1,2}\ensuremath{\rightarrow}\ensuremath{\gamma}J/\ensuremath{\psi}$ and search for the transition ${\ensuremath{\eta}}_{c}(2S)\ensuremath{\rightarrow}\ensuremath{\gamma}J/\ensuremath{\psi}$

Measurement of higher-order multipole amplitudes in $ψ (3686) \to γ χ_{c 1, 2}$ with $χ_{c 1, 2} \to γ J / ψ$ and search for the transition $η_{c} (2 S) \to γ J / ψ$

M. Ablikim et al. (BESIII Collaboration)

Phys. Rev. D 95, 072004 – Published 6 April 2017

Abstract

Using 106 $\times 1 0^{6}$ $ψ (3686)$ events collected with the BESIII detector, we measure multipole amplitudes for the decay $ψ (3686) \to γ χ_{c 1, 2} \to γ γ J / ψ$ beyond the dominant electric-dipole amplitudes. The normalized magnetic-quadrupole (M2) amplitude for $ψ (3686) \to γ χ_{c 1, 2} \to γ γ J / ψ$ and the normalized electric-octupole amplitudes for $ψ (3686) \to γ χ_{c 2}$ , $χ_{c 2} \to γ J / ψ$ are determined. The M2 amplitudes for $ψ (3686) \to γ χ_{c 1}$ and $χ_{c 1, 2} \to γ J / ψ$ are found to differ significantly from zero and are consistent with theoretical predictions. We also obtain the ratios of M2 contributions of $ψ (3686)$ and $J / ψ$ decays to $χ_{c 1, 2}$ , $b_{2}^{1} / b_{2}^{2} = 1.35 \pm 0.72$ and $a_{2}^{1} / a_{2}^{2} = 0.617 \pm 0.083$ , which agree well with theoretical expectations. By considering the multipole contributions of $χ_{c 1, 2}$ , we measure the product branching fractions for the cascade decays $ψ (3686) \to γ χ_{c 0, 1, 2} \to γ γ J / ψ$ and search for the process $η_{c} (2 S) \to γ J / ψ$ through $ψ (3686) \to γ η_{c} (2 S)$ . The product branching fraction for $ψ (3686) \to γ χ_{c 0} \to γ γ J / ψ$ is $3 σ$ larger than published measurements, while those of $ψ (3686) \to γ χ_{c 1, 2} \to γ γ J / ψ$ are consistent. No significant signal for the decay $ψ (3686) \to γ η_{c} (2 S) \to γ γ J / ψ$ is observed, and the upper limit of the product branching fraction at the 90% confidence level is determined.